At present radio communication systems such as portable telephone systems and radio LANs (Local Area Networks) are widely used. With radio communication a sending apparatus modulates data and outputs it as a transmitted signal. A receiving apparatus demodulates the received signal and reconstructs the data. There are various modulation methods and one of them is multilevel modulation. With the multilevel modulation the amplitude and phase of a signal is controlled. By doing so, a plurality of information bits can be mapped to a unit signal interval (symbol). With 16QAM (Quadrature Amplitude Modulation), for example, 4 bits can be mapped to one symbol. With 64 QAM 6 bits can be mapped to one symbol.
With the multilevel modulation there are cases where the aptness of an error to occur depends on the position of a bit mapped to a symbol. With 16 QAM, for example, it is assumed that 2 higher-order bits are assigned for identifying first through fourth quadrants on an I-Q plane and that 2 lower-order bits are assigned for identifying 4 points in each quadrant. As a result, the difference in amplitude or phase between symbols which differ in 2 higher-order bits is comparatively large and an error is unapt to occur. On the other hand, the difference in amplitude or phase between symbols which are equal in 2 higher-order bits and which are different only in 2 lower-order bits is comparatively small and an error is apt to occur. That is to say, the aptness of an error to occur differs between the 2 higher-order bits and the 2 lower-order bits.
Hierarchical modulation is known as a technique using this property of the multilevel modulation. With the hierarchical modulation plural kinds of information bits can be mingled and mapped to a symbol. For example, if image data is transmitted, a method for mapping an information bit indicative of data, such as low resolution data, which is greatly influenced at the time of being reproduced to a position where an error is unapt to occur and mapping an information bit indicative of data, such as high resolution data, which is slightly influenced at the time of being reproduced to a position where an error is apt to occur is known (see, for example, M. Morimoto, M. Okada, and S. Komaki, “A Hierarchical Image Transmission System in a Fading Channel”, Proc. IEEE International Conference on Universal Personal Communications (ICUPC '95), pp. 769-772, 1995; S. Pietrzyk, and G. J. M. Janssen, “Subcarrier and Power Allocation for QoS-aware OFDMA System Using Embedded Modulation”, Proc. IEEE International Conference on Communications (ICC 2004), Vol. 6, pp. 3202-3206, 2004; P. K. Vitthaladevuni, and M-S. Alouini, “A Recursive Algorithm for the Exact BER Computation of Generalized Hierarchical QAM Constellation”, IEEE Transactions on Information Theory, Vol. 49, No. 1, pp. 297-307. 2003; K. Ramchandran, A. Orteg, K. M. Uz, and M. Vetterli, “Multiresolution Broadcast for Digital HDTV Using Joint Source/Channel Coding”, IEEE Journal on Selected Areas in Communications, Vol. 11, No. 1, pp. 6-22, 1993; M. B. Pursley, and J. M. Shea, “Adaptive Nonuniform Phase-Shift-Key Modulation for Multimedia Traffic in Wireless Networks”, IEEE Journal on Selected Areas in Communications, Vol. 18, No. 8, pp. 1394-1407, 2000; D. I. Kim, “Two-Best User Scheduling for High-Speed Downlink Multicode CDMA with Code Constraint”, Proc. IEEE Global Telecommunications Conference (Globecom 2004), pp. 2659-2663, 2004; M. J. Hossain, M-S. Alouini, and V. K. Bhargava, “Rate Adaptive Hierarchical Modulation-Assisted Two-User Opportunistic Scheduling”, IEEE Transactions on Wireless Communications, Vol. 6, No. 6, pp. 2076-2085, 2007; and A. Chindapol, and J. Ritcey, “Bit-Interleaved Coded Modulation with Signal Space Diversity in Rayleigh Fading”, Proc. IEEE Asilomar Conference on Signals, Systems and Computers, (ACSSC 1999) Vol. 2, pp. 1003-1007, 1999.
By the way, it is assumed that, in a communication system in which a sending apparatus transmits data to a plurality of receiving apparatus, the data the destinations of which are the plurality of receiving apparatus is hierarchically modulated. However, if the hierarchical modulation is adopted, there are various choices such as which multilevel modulation method (16QAM or 64QAM, for example) is used for each channel, which bit is assigned to data the destination of which is each receiving apparatus, and the like. Accordingly, there are problems about how to select a proper hierarchical modulation method.